1. Field of the Invention
The present invention relates generally to the configuration of a medical implant that stimulates a physiological system of a recipient, and more particularly, to fitting of a cochlear implant.
2. Related Art
There are many medical implants that deliver electrical stimulation to a recipient for a variety of therapeutic benefits. For example, cochlear implants, such as those manufactured by Cochlear Limited, Sydney, Australia, have been developed to provide persons suffering from sensorineural hearing loss with the ability to perceive sound. The hair cells of the cochlea of a normal healthy ear convert acoustic signals into nerve impulses. People who are profoundly deaf due to the absence or destruction of cochlea hair cells are unable to derive suitable benefit from conventional hearing aid systems. Cochlear implants have been developed to provide such persons with the ability to perceive sound.
Cochlear implants typically comprise external and implanted or internal components that cooperate with each other to provide sound sensations to the recipient. The external component traditionally includes a microphone that detects sounds, such as speech and environmental sounds, a speech processor that selects and converts certain detected sounds, particularly speech, into a coded signal, a power source such as a battery, and an external transmitter antenna.
The coded signal output by the speech processor is transmitted transcutaneously to an implanted receiver/stimulator unit. This transcutaneous transmission occurs via the external transmitter antenna which is positioned to communicate with an implanted receiver antenna disposed within the receiver/stimulator unit. This communication transmits the coded sound signal while also providing power to the implanted receiver/stimulator unit.
The implanted receiver/stimulator unit also includes a stimulator that processes the coded signal and outputs an electrical stimulation signal to an intra-cochlear electrode assembly. The electrode assembly typically has a plurality of electrodes that apply electrical stimulation to the auditory nerve to produce a hearing sensation corresponding to the original detected sound.
Following surgical implantation of the internal components (including the receiver/stimulator unit and intra-cochlear electrode assembly), the cochlear implant system must be configured (or fitted) for each individual recipient. This configuration procedure is normally carried out by an audiologist several weeks after implantation.
An important aspect of this configuration procedure is the collection and determination of a number of recipient-specific input configuration variables that are required for normal operation of the cochlear implant system. Two of the input configuration variables that require determination include the threshold level of electrical stimulation (known as a T level), and the maximum comfort level of electrical stimulation (known as a C level) for each electrode stimulation channel. Together, the T and C levels define a “dynamic range” of electrical stimulation for each electrode channel.
The T level is defined as the level at which the recipient first identifies sound sensation, and is the lowest level at which the recipient hears the stimulus. The C level sets the maximum allowable stimulation level for each electrode and is defined as the maximum stimulation level that does not produce an uncomfortable loudness sensation for the recipient.
Conventionally, T and C levels are manually determined by a clinician working together with the recipient. For each stimulation channel of the implant, the clinician applies stimulation pulses and then receives an indication from the recipient as to the level and comfort of the resulting sound.
Referring to FIG. 2, there is shown graphically the settings 200 for the T and C levels for each electrode 210 in a 22 electrode system as determined by a clinical fitting procedure. The set of T levels and C levels across the electrode array constitute a T level profile 220 and a C level profile 230, respectively. If a T level is set too low, then stimuli are applied which cannot be perceived. If the C level is set too high, then the recipient may be overstimulated, leading to pain and possible injury to the recipient.
This post-operative configuration or fitting process can be extremely time consuming. In locations where there is a lack of adequate audiological infrastructure and/or trained clinicians, the cochlear implant may not be optimally fitted for some recipients. Additionally, since this post-operative configuration process relies on subjective measurements, children, pre-lingually deaf or congenitally deaf recipients are often unable to provide an accurate impression of the hearing sensation resulting from the stimulation test pulses. This further complicates the fitting process, potentially resulting in a cochlear implant that is not optimally fitted.
Referring now to FIG. 3, in an attempt to improve the efficiency of the fitting process one approach has been to base the shape of one type of profile on the shape of another type of profile. An example of this approach is depicted in settings 300, wherein the shape of the C level profile 330 (i.e. the electrode to electrode variation in level) is first matched to the shape of the corresponding T level profile 320. Following this matching process, C level profile 330 is then shifted to achieve comfortable loudness for the recipient.
Accordingly, the C levels 330 of the electrodes 310 are based only on the shape of T level profile 320 and a shift measure ΔS. Thus the relative differences in stimulus level between adjacent electrodes for the C level profile 330 are maintained in accordance with the T level profile 320. Only the overall mean stimulus level then needs to be manipulated by changing ΔS to achieve comfortable loudness.
In another approach, objective measures of the auditory system are employed to simplify the task of configuring the cochlear implant. Cochlear implant objective measures include those physiological signals that are related to the auditory system, such as the electrically evoked auditory nerve action potential (ECAP), the auditory brain stem response (EABR) or the stapedius reflex (ESR). These physical characteristics of the auditory system are described in detail in Cullington H. E., Cochlear Implants: Objective Measures, London, Whurr Publishers, 2003.
These and other physical characteristics can be measured by employing a cochlear implant system such as the Cochlear™ Nucleus™ system employing Neural Response Telemetry (NRT) where the minimum stimulus level required to evoke a detectable response can be determined on each electrode. The telemetry mode enables a telemetry facility within the cochlear implant to measure various physical characteristics of the recipient's physiological system.
In this telemetry mode, the implanted electrode array is used to provide test stimuli and to then measure a neural response of the recipient's physiological system. The test stimuli are delivered by means of a number of “stimulation channels.” For example, the delivery of a stimulation current between two particular electrodes of the electrode array may be defined as stimulation via channel 1. Similarly, other combinations of electrodes involved in stimulation delivery will also define other stimulation channels. An exemplary telemetering arrangement is described in U.S. Pat. No. 5,758,651. The profile of these objectively measured physical thresholds constitutes another type of profile that may be replicated.
Referring now to FIG. 4, there is shown graphically the settings 400 for a T level profile 420 and C level profile 430 that have been matched to an objective threshold profile 440. In this example, an ECAP threshold has been measured for each electrode 410, resulting in ECAP threshold profile 440. The T level profile 420 and C level profile 430 are then set at constant offsets ΔS1 and ΔS2 from ECAP threshold profile 440. These offsets are determined by the clinical measurement on a single channel, typically a mid-frequency channel, and then applied accordingly across all channels.
However, while this approach has the effect of reducing the time required for the clinical fitting of a cochlear implant, it clearly suffers from a number of disadvantages. The primary disadvantage is the assumption that the shape of the profile determined at a first level (typically an objectively measured or clinically determined threshold level) will relate to the shape of the profile at a second level at a different overall mean or stimulus level. This will not take into account variations in the auditory system that depend on stimulus level.